I'm on Ubuntu 11.10 and I wanna know which version (OpenJDK or Sun's one or whatever) is the best to develop applications and use it with Eclipse, and how to install it. Thank you.
You can have various JDK side-by-side in the same machine. This is needed because sometimes you are on a 64-bit machine but need to run a specific 32-bit stuff. Also there are others JVM availables, some (open source): JamVM, Cacao, JatoVM. They are usually made for a specific niche and unfortunately are not complete as the reference JVM (from Sun/Oracle). The official JVM (from Oracle/Sun), due to it's completeness, is your best bet in almost all occasions.
Since you pretend to develop Android applications I highly recommend using the Sun (Oracle) 1.7 i386 JDK. Why? Not only it's the official JVM but due to android not having a 64-bit SDK.
This link provides a step-by-step installation of the Sun (Oracle) 1.7 JDK:
Sun's Oracle's JDK... it depends on what kind of applications you're going to develop. There is plenty of Java software that currently won't play well with OpenJDK. (eg. Android SDK, IntelliJ Idea, PyCharm, etc).